French Open champion Jelena Ostapenko and Kristina Mladenovic were among the seeded casualties in the Rogers Cup first round in Toronto on Monday.
Ostapenko backed up her stunning triumph at Roland Garros by reaching the quarter-finals at Wimbledon, but was stunned in the first round as Varvara Lepchenko came from behind to win.
Mladenovic lost in straight sets to Barbora Strycova, but there were no such problems for Venus Williams, Petra Kvitova and Dominika Cibulkova, who all progressed.
The other seed to play on Monday, Anastasija Sevastova, saw off Lauren Davis in three sets while Sloane Stephens and Daria Gavrilova made their way through.
LEPCHENKO FIGHTBACK STUNS OSTAPENKO, KIKI CRASHES OUT
Ostapenko seemed to be en route to an easy win after taking the first set 6-1, only for Lepchekno - ranked 56 places below her in 68th - to claim the next two sets on tie-breaks and prevail.
Mladenovic was eliminated by Barbora Strycova in under 90 minutes, losing 6-2 6-3.
The French 13th seed could not gather any sort of rhythm and Strycova took full advantage. Mladenovic produced six aces and a first-serve rate of 61 per cent, but Strycova punished anything else, winning 84 per cent of return points on second serve.
Strycova raced to a 5-0 lead in the first set and the world number 26 never looked back, leaving Mladenovic still waiting to win a singles match in either Montreal or Toronto.
KVITOVA BOUNCES BACK
After a stunning 6-2 6-0 defeat to Cici Bellis last Friday in Stanford, Petra Kvitova recovered with a relatively comfortable 6-1 7-6 (7-5) victory over Carla Suarez Navarro.
In contrast to her loss to Bellis, Kvitova was able to open the shoulders and hit through the ball, producing some stunning winners in the process.
The Czech 14th seed will now face Sloane Stephens, who dropped the opening set against Yulia Pulintseva, before storming back to win 6-7 (4-6) 6-0 6-4.
RAIN DELAY FAILS TO TRAP VENUS
Venus Williams had to see off a lengthy rain delay and a gritty performance from qualifier Irina-Camelia Begu, eventually winning 6-1 3-6 6-3.
Play resumed with Begu serving at deuce at 1-2 in the final set and, though Williams immediately broke, the Romanian showed great resilience to strike back and level at 3-3.
But this year's Australian Open and Wimbledon finalist was able to regroup and claimed the final three games of the match.
Despite a career spanning over two decades, the win marks Williams' first in Toronto.